Improved efficacy αvβ6-targeted albumin conjugates by conjugation of a novel auristatin derivative

Cellular handling of drug delivery preparations en route to the lysosomal compartment has been extensively studied, but little is known about cellular handling of drugs subsequent to their release from the delivery system. We studied a series of closely related drug targeting conjugates, consisting of albumins equipped with alpha(v)beta(3)-selective RGD-peptide homing ligands, PEG stealth domains, and either the antitubulin agent monomethyl auristatin E (MMAE) or a new F-variant (MMAF). Since MMAF has a C-terminal charge, this compound is potentially less prone to passive redistribution after its release from the carrier. We demonstrate that RGD-peptide-equipped albumin conjugates with MMAF were indeed more potent than MMAE conjugates, in killing both alpha(v)beta(3)-positive tumor cells and proliferating endothelial cells. Efficacy increased more in tumor cells than in endothelial cells, suggesting different drug redistribution behavior for the two cell types. Binding affinity and uptake of the conjugate and the cellular handling of released drug contributed to the final efficacy of drug-carrier conjugates, highlighting the importance of all aspects to be carefully considered in the design of targeted drug delivery preparations.